Plasma-based material synthesis and processing techniques are used in a large number of industrial applications. In recent years, advances in plasma electrochemistry have opened the possibility of synthesizing nanoparticles (and microparticles) by projecting an atmospheric-pressure plasma at the surface of a liquid containing metal ions from which the particles to be synthesized are composed (see, e.g., W.-H. Chiang et al., Plasma Sources Sci. Technol., vol. 19, no. 3, p. 034011, 2010; S. W. Lee et al., Catal. Today, vol. 211, p. 137-142, 2013). Metal nanoparticles (and microparticles) can be used in a number of applications, including catalysis, biomedical imaging, radiotherapy, optics and optoelectronics, paints, inks, coatings, and nanomedicine.
It is now generally recognized that plasma-liquid electrochemistry may allow nanoparticles to be synthesized not only more rapidly and efficiently than with conventional colloidal chemistry techniques, but also with environmentally-safer processes that limit the use of toxic chemicals as reducing agents. This is the case for nanoparticle synthesis processes involving metal ion reduction, where using plasmas allows the consumption of toxic or contaminating reducing agents (e.g., sodium citrate or sodium borohydride) to be decreased, or even avoided. In addition, by limiting the number of chemical species introduced in the metal precursor bath, nanoparticle suspensions with simpler chemical compositions and, in turn, improved colloidal stability can be produced.
These anticipated advantages have spawned a growing interest in developing atmospheric plasma-based techniques for synthesizing nanoparticles. One approach that has been investigated is based on atmospheric-pressure plasma reactors having submillimeter-sized hollow cathodes. However, while this approach may provide certain advantages, it also suffers from a number of drawbacks and limitations, among which are the practical limits on the size of the treatment area over which plasma homogeneity can be achieved, and the resulting difficulty of scaling up the plasma reactors to high-volume, continuous-flow and/or automated production.
Accordingly, various challenges still exist in the development of atmospheric plasma-based metal nanoparticle synthesis techniques capable of being scaled up to larger treatment areas while preserving adequate plasma homogeneity.